The present invention generally relates to a plasma pressure control assembly and, in particular, relates to one such assembly having a wall surrounding an RF head assembly and extending closer to the surface to be etched than the RF head.
In one type of plasma assisted chemical etching reactor system, an RF head assembly having an RF electrode associated therewith extends into a reaction chamber such that a plasma can be created between a surface to be etched and the RF electrode. Further, in order to more selectively control the etching of the surface, the RF head, and hence the plasma footprint, is made small with respect to the surface and relative movement is introduced between the RF head and the surface.
In order to provide a uniform plasma, the entire chamber within which the etching occurs must be maintained at a particular pressure. That is, the plasma, in order to have and maintain desired characteristics, such as diameter, etchant gas concentration, and so forth, should be formed in a known and controlled ambient. Variations in the ambient during the plasma etching process can result in variations in the plasma itself and, thus, a departure from the intended etching of the surface.
In general, one mechanism for controlling the etching plasma within the reactor chamber is by regulating the static background pressure of the ambient to values ranging, for example, from 1 to 10 Torr.
One particular technique for controlling the static background pressure of the reaction chamber is by introducing a make-up gas into the reactor chamber. In conventional chambers, the amount of make-up gas needed is dependent on, among other factors, the overall volume of the reaction chamber. The make-up gas is, typically, the same composition as the reactant gas supplied to the etching plasma. As plasma reactor chambers become larger with respect to the size of the RF electrode, the required volume of make-up gas needed can exceed that needed for supporting the actual plasma. Further, the flow rate of the make-up gas must be increased in order to ensure a constant static background pressure within the reactor chamber. In addition, the increased volume of the reaction chamber inherently permits the creation of pressure gradients within the chamber. Thus, the supplying of make-up gas and its effects becomes a process factor that cannot be ignored.
In addition, the increased volume and flow rate of make-up gas used during the etching process causes particulates to become entrained and distributed about the chamber in accordance with the flow patterns established by the combination of the vacuum system, the flow of the make-up gas and the plasma reactants. These particulates become the stirred contaminants during pump-down and some may chemically interact with the chamber itself and thus result in further contamination.
In light of the above, it is clear that a plasma pressure control assembly is needed that more precisely and economically controls the pressure of an etching plasma.